Non-coding RNAs (ncRNAs) are critical regulators of gene expression. The best-characterized ncRNAs that are processed and function via the RNAi pathway are microRNAs (miRNAs). It has been estimated that expression of at least 30% of mammalian protein-encoding genes is regulated, to some extent, by microRNAs (miRNAs) via the RNAi pathway. Primary miRNA transcripts undergo multi-step enzymatic processing to generate the mature form via a precursor intermediate. Mature miRNAs exert their effect on endogenous gene expression through the formation of a ribonucleoprotein (RNP) complex that uses the miRNA as a guide for the sequence alignment of the miRNA-RNP complex with a transcript, usually within the 3'UTR region. The molecular mechanism(s) by which miRNAs modulate protein expression are multifarious but include altered mRNA stability and translational repression. Many biological processes have been implicated as being regulated by miRNAs including development, cell differentiation and proliferation, and metabolism. The altered biogenesis, deregulated expression, and disruption of specific mRNA-miRNA interactions have all been implicated as contributing to the development and maintenance of human cancers. Our research studies are focused on the hypothesis that the regulation of gene expression by miRNAs is altered in cancer. For example, we recently established that the receptor tyrosine kinase AXL as a functionally relevant target of the p53 regulated miRNA, miR-34a, in breast cancer. We have two ongoing studies: (1) Investigation of a series of miRNAs we identified within a region (8q24) associated with genomic instability that includes the long non-coding RNA (lincRNA), PVT1, downstream of the proto-oncogene MYC. These small non-coding RNA species have been annotated as microRNAs (miR-1204-1208) and we, and others, have shown that at least some of these are functionally active and that at least one, miR-1204, is responsive to regulation by p53. Current research is focused on examining the over-expression of miR-1204 in a panel of mouse cell lines representing evolving stages of B cell development to determine what role this miRNA may play in normal B cell development and lymphoma. (2) We are also examining the regulatory relationship of miRNAs and oncogenic transcription factors. There is evidence that many transcription factors and miRNAs act in concert to directly or indirectly regulate each others expression. Disruption of this regulatory balance, as a consequence of the deregulated or aberrant expression of transcription factors and miRNAs is clearly an important mechanism underlying tumorigenesis. We are investigating the regulation of expression by miRNAs of aberrant transcription factors formed as a consequence of genetic translocations. Currently we are using the EWS-FLI1 fusion transcription factor associated with Ewing sarcoma as a model system.